Your most disappointing purchase or audition?

Other than reliability/service issues (decomposing surrounds on Zingali speakers and horrible Audible Illusions repair service come to mind) I don't hink that I've ever bought anything that I later really regretted (and I have bought a lot over the years). However, I've definitely heard some clunkers on demo:

I remember hearing the original Martin-Logan Sequel at IIRC Innovative Audio in Brooklyn Heights about a million years ago. That was some really bad integration of a dynamic woofer and a panel. It might have been one of the first pairs manufactured and the underdamped woofers were just a mess. I believe that later examples of that model were much better sounding (there was also a Sequel II at some point that addressed the issue). I also heard a demo of the big B&O room corrected speakers in their Beverly Hills showroom +/- ten years ago that was so bad that I'm still left wondering if one or both of the speakers was defective.

Cary SLP05. Spent big dollars (for me) to purchase it used ($4k) and I never could get it to sound up to its reputation even after rolling tubes. In fairness, the unit's performance may have been a simple case of poor system synergy in my setup.

Also, Cary 303/300, no better than a boat anchor imo. No benefit of the doubt here though.

One man's meat is another man's poison.

Magico S3. It was in a dedicated room with McIntosh gear. It was OK, but not worth the money or the hype. No passion to the sound.

say the Pass Labs X 250.5. I used it in single ended mode, which I was told later was not optimum, although I never read that in all of my pre-purchase research. I bought it and auditioned it with 5 different speakers in my system, and in each case, the sound was thin and electronic. It was a really shocking disappointment.

Here are some units listed which are absolutely the opposite from sound quality what is written in their "reviews".
The golden rule is, EVERY unit gets a positive review, be it awful sounding or good sounding. The vocabulary will be the same (and is identical). Blind buy is really dangerous, the majority is really down at the docks....

When I did own Pass units I was sitting in a Demo listening to the latest Pass amps, at that time it was the "X" series and it was exactly as described. Ultra boring for me, so unbelievable dead that I could not believe that the same guy made the outstanding Aleph amps. Anyway, left side was a Pass Fan and he asked me what I think about those super-duper-reviewed amps, I lied and said "amazing". He loved me from one second to the next, smiled and told me his impressions about the "super soundstage", the "holographic detail" and so on and that they are M-U-C-H better than the model before (which I also knew very well). When I listened his explanations I really was interested to know what he did smoke that made him speaking about abilities which were simply not present. I mean, I was right beside him.

You see, there is a Fangroup for everything out there.